This application claims benefit of priority under 35 U.S.C. 120 to Japanese Patent Application No. P11-134972 filed in the Japanese Patent Office on May 14, 1999, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to an electron-emitting device to be applied to images units and electron beam exposure systems. The present invention relates also to a process for producing the same.
2. Description of the Related Art
Application of a high electric field (on the order of 107 V/cm) to the surface of a metal or a semiconductor causes electrons to be emitted into a vacuum through tunneling to the vacuum level. Electron emission of this kind is generally called field emission.
A field emission type cold cathode offers the advantage of emitting a larger number of electrons per unit area than emitted by a hot cathode. In other words, electron emission from cold cathodes can be as large as is 107 to 109 amperes per cm2, whereas that from a hot cathode is limited to tens of amperes per cm2. Therefore, a field emission type cold cathode is particularly useful for the miniaturization of vacuum electron devices.
An actual example of a miniaturized vacuum device, or vacuum microelectronic device, that employs a cold cathode was reported by Shoulders (Adv. Comput. 2 (1961) 135.) This publication discloses a process for producing a device of the size of 0.1 micron, and also covers a process for producing a minute diode of a field emission type by using the disclosed device.
Spindt et al (J. Appl. Phys. 39 (1968) 3504.) reported a process for producing by thin-film technology a large number of gated cold cathodes., or triodes, of micron size arranged in an array structure on a substrate. Since then, many reports have appeared in this field.
The cold cathode proposed by Spindt et al is designed such that an electric field is concentrated at the sharp tip of a minute pyramidal emitter of micron size, and the field emission of electrons is controlled by a gate electrode located nearby the tip.
The Spindt-type device is provided with a gate having an opening right above the emitter and an anode placed above the emitter. The number of electrons emitted toward this anode can be controlled by the gate-emitter voltage.
Many other electron emitting devices of similar structure have been reported. They are produced by etching of silicon or by molding and are of a xe2x80x9cvertical structure,xe2x80x9d in which the emitter and gate are arranged vertically with respect to the substrate.
By contrast, those of a so-called xe2x80x9chorizontal structurexe2x80x9d are also reported. These have a pair of electrodes arranged on a substrate, one functioning as the emitter and the other functioning as the gate.
The horizontal type device is inferior to that of a vertical type in electron emitting efficiency. However, the former offers the advantage of being produced easily especially in the case where a number of elements are arranged in a large area.
Incidentally, electron emitting efficiency is defined as the ratio of current reaching the anode to current flowing through all the elements. In other words, it is the quotient obtained by dividing the number of electrons leaving the emitter and reaching the anode by the total number of electrons entering the emitter. Electrons emitted by the emitter partly reach the anode, and partly are absorbed by the gate. The greater the value of electron emitting efficiency, the larger the number of electrons reaching the anode or the larger the amount of current.
One example of the horizontal type device shown in FIG. 8, reported in J. Vac. Sci. Technol. B13 (1995) 465, consists of a silicon substrate 701, an insulating layer 702 of SiO2, and an H-shaped metal thin film 703, arranged sequentially on top of each other. The metal thin film has a minute narrow gap 705 (about 2 xcexcm wide) formed by a focused ion beam 704. Thus, a pair of electrodes (an emitter 706 and a gate 707) is formed, with the minute narrow gate gap 705 interposed between them. An anode is placed above the paired electrodes a certain distance away from and parallel to the silicon substrate 701. Electrons are emitted by applying a potential difference across the emitter 706 and the gate 707, and partly recovered at the anode.
One effective way to reduce the voltage for electron emission in the horizontal type device is to sharpen the tip of the electrode, or to narrow the gap between the electrodes down to a sub-micron level (below a micron) between the electrodes.
Many other devices having similar configurations have also been fabricated by a process called xe2x80x9celectroforming.xe2x80x9d In this process, an electric current is passed through the film, and the film suffers Joule heating which forms cracks that separate electrodes in the film. The electron-emitting device of this kind is sometimes called a xe2x80x9csurface conduction type electron-emitting device.xe2x80x9d
An example of such device is disclosed by M. I. Elinson, Radio Eng. Electron Phys., 10. 1290 (1965). The advantage of this device is the ease with which it can be produced. Elinson""s devices in which the film is formed by vacuum deposition, however, suffers the disadvantage of being unstable in action and short in life. This disadvantage was overcome by the device shown in FIG. 9, which is disclosed in Japanese Patent No. 2-646235. This device, formed from a fine particle film, is greatly improved in reliability. It has electrodes 1101 and 1102 on a substrate and it also has a fine particle thin film between these electrodes 1101 and 1102. A minute gap 1105 is formed by the electroforming. This minute gap 1105 separates the emitter 1103 and the gate 1104 from each other. Fine particles 1106 are partly exposed on the edges of the minute gap 1105.
The horizontal type device as mentioned above is inferior to that of a vertical type in electron emitting efficiency. One reason for this is reported by A. Asai, SID 97 DIGEST, 127. The device described proposed by Asai et al. is shown in FIG. 10. It has wiring 12204 formed on a substrate. The wiring is covered with a thin film, which has a minute gap 12201 formed at the center thereof. The emitter 12202 and the control electrode 12203 are formed, having this minute gap 12201 interposed between them. The anode 12206 is placed a certain distance away and above the emitter 11202 and the control electrode 12203.
As the emitter 12202 emits electrons into a vacuum, the emitted electrons fly mostly toward the control electrode 12203 and partly toward the anode 12206. Most of the emitted electrons reach the control electrode 12203. Electrons are partly disturbed by the control electrode 12203, and are caused to move toward the anode 12206. Almost all of the electrons are recovered at the control electrode 12203. On the upper side of the control electrode near the emitter is a region in which there exists an upward electric field. Electrons which enter this region are accelerated toward the control electrode. The problem with this is that most of the emitted electrons do not reach the anode 12206. In addition, the flow of electrons from the emitter 12202 to the control electrode 12203 causes heat generation, and wastes electric power.
One known way to improve the device efficiency is to increase the number of electrons scattered or reflected by the control electrode. For example, Japanese Patent Laid-open No. 265899/1998 discloses a control electrode which is provided with an easily oxidizable material to increase the reflection of electrons.
Also, Japanese Laid-Open Patent No. 231674/1994 discloses a control electrode which is provided with electrically conductive ultra fine particles having a radius equal to the mean free path of electrons. The film of ultra fine particles readily changes the direction of the electrons colliding with it. The result is a reduction in number of electrons captured by the control electrode.
Another method of increasing the emission efficiency is disclosed in Japanese Patent Laid-open No. 8221/1998. This method consists of correcting the electric field on the control electrode, thereby decreasing the number of electrons falling on the control electrode. The object is achieved by providing a third electrode next to the emitter or control electrode so as to control the electric field on the control electrode. The disadvantage of this method is that electrons fall on the correction electrode if the correction electrode for correcting the electric field is not provided adequately, with the result that efficiency is not improved. Therefore, placement of the correction electrode is restricted. Moreover, for a sufficient improvement in efficiency, it is necessary to place the correction electrode near the point of electron emission and to apply the an voltage in excess of about 100 V. This leads to device instability.
As mentioned above, the electron emission horizontal type device is simpler in structure than that of a vertical type, and hence it is easier to produce. On the other hand, it has a low electron emission efficiency. Various attempts have been made to improve the electron emission efficiency of the horizontal type device, e.g., by increasing the scattering or reflection of electrons on the surface of the control electrode or by correcting the distribution of the electric field. Nevertheless, further improvement from the standpoint of better efficiency and device stability is desired.
Accordingly, the present invention provides a horizontal type electron emission device including a control electrode and a third secondary-electron emitting material. The control electrode supplies a sufficient amount of scattered electrons, and causes electrons to collide with the third secondary-electron emitting material, thereby improving electron emission efficiency.
Therefore, according to the present invention, it is desirable that the electrons are efficiently scattered by the surface of the control electrode, and that electrons are emitted toward the anode. To that end, the secondary-electron emission takes place efficiently on the surface of the control electrode.
Thus, the present invention is directed to a horizontal type electron-emitting device including a low-potential electrode and a high-potential electrode formed separately on a substrate and on an electron emitting part formed between said electrodes. A secondary-electron emitting material, capable of emitting secondary electrons more efficiently than the material of the high-potential electrode, is exposed on a part of the substrate in the region from the electron-emitting part to the high-potential electrode.
Owing to the secondary-electron emitting material which is exposed in the vicinity of the electron-emitting part and the high-potential electrode, the device of the present invention efficiently emits secondary electrons upon collision of electrons emitted from the electron emitting part.
The present invention is also directed to a horizontal type electron emitting device, which further includes an auxiliary electrode formed near the high-potential electrode on the substrate, with an insulating layer or high-resistance layer interposed between these electrodes. The voltage applied to the auxiliary electrode is higher than that applied to the high-potential electrode. This structure causes electrons emitted from the electron emitting part to collide with the auxiliary electrode, so that secondary electrons are emitted efficiently from the auxiliary electrode.
According to the present invention, a secondary-electron emitting material is exposed on the upper surface of the auxiliary electrode, and a properly controlled voltage is applied to the high-potential electrode and the auxiliary electrode. This causes electrons emitted from the electron emitting part to collide with the auxiliary electrode, so that it is possible to double multiply the number of electrons for emitting secondary electrons from the auxiliary electrode.
The secondary-electron emitting material includes at least one of LiF, CaF, AlN, BN, B, Bi, Ga, BaO, and MgO.
According to the present invention, the high-potential electrode and its vicinity are covered with a film containing the above-mentioned secondary-electron emitting material. This causes secondary electrons to be emitted efficiently no matter where electron collision takes place.
According to the present invention, a secondary-electron emitting material in the form of fine particles is exposed in at least one part of the region from the electron emitting part to the high-potential electrode. This improves the efficiency of secondary-electron emission.
The present invention is also directed to a process for producing an electron emitting device, including forming a low-potential electrode, a high-potential electrode, and an electron emitting part separately on a substrate, and applying a voltage across the high-potential electrode and the low-potential electrode in a gas containing a boron compound, so that electrons are emitted from the electron emitting part, decompose the gas, and form a BN film on the upper surface of the electron emitting part and its vicinity.
The BN film formed in the vicinity of the electron emitting part improves the efficiency of electron emission from the electron emitting part. The boron compound from which the BN film is formed includes, for example, boranes, amineboranes, aminoboranes, pyridineboranes, piperazineboranes, alkylboranes, boric acid esters (such as alkoxyboranes and arylborane), and cyclic compounds (such as borazine) having Bxe2x80x94N bonds.
The electron-emitting part may be formed from a material containing boron. In this case, the process starts with forming the low-potential electrode, the high-potential electrode, and the electron-emitting part on the substrate. Then, heat treatment is carried out in a nitrogen or a nitrogen-containing atmosphere, so as to form a BN film on the upper surface of the electron-emitting part and its vicinity.
With such simple treatment, it is possible to form a BN film in the periphery of the electron-emitting part.
The present invention further provides a field emission element including a substrate; a first electrode on the substrate; a second electrode separated from the first electrode on the substrate; a coating layer having a smooth surface provided on the second electrode, wherein the coating layer is formed by a secondary-electron emitting material.
An accelerating energy of an electron formed by an electric potential difference between the first electrode and the second electrode may substantially equal to EPE(m).
Still further, the present invention provides a field emission element including a substrate; a first electrode on the substrate; a second electrode separated from the first electrode on the substrate; a third electrode separated from the second electrode on the substrate, and the second electrode being put between the first electrode and the third electrode; and a first secondary-electron emitting material on the second electrode.
The first electrode and the second electrode may engage each other.
The first secondary-electron emitting material can coat the second electrode smoothly.
The first secondary-electron emitting material may be particles.
An accelerating energy of an electron formed by an electric potential difference between the first electrode and the second electrode may be between EPE(I) and EPE(II). An accelerating energy of an electron formed by an electric potential difference between the first electrode and the second electrode may substantially equal to EPE(m).
The field emission element may further include a second secondary-electron emitting material on the third electrode.
The second secondary-electron emitting material may coat the third electrode smoothly. The second secondary-electron emitting material may be particles.
A maximum of a secondary electron increasing ratio of the first secondary-electron emitting material may be smaller than a maximum of a secondary electron increasing ratio of the second secondary-electron emitting material. An EPE(I) of the first secondary-electron emitting material may be smaller than an EPE(I) of the second secondary-electron emitting material. An accelerating energy of an electron formed by an electric potential difference between the second electrode and the third electrode may be between EPE(I) and EPE(II). An accelerating energy of an electron formed by an electric potential difference between the second electrode and the third electrode substantially equals to EPE(m).
The field emission element may further include an insulator between the first electrode and the second electrode.
The field emission element may further include an insulator between the second electrode and the third electrode.
The field emission element may further include a third secondary-electron emitting material on the insulator.